Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam, an LED source, or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two-component and single-component developer materials are commonly used for development. A typical two-component developer comprises magnetic carrier granules having toner particles tribo-electrically charged and adhering thereto. A single-component developer material typically comprises toner particles. Toner particles are attracted to the latent image, forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet. Finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process given above can be modified to produce color images. For example, a color electrophotographic marking process, called image-on-image (IOI) processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While the IOI process provides certain benefits, such as a compact architecture, there are several challenges to its successful implementation. For instance, the viability of printing system concepts such as IOI processing requires development systems that do not interact with a previously toned image. Since several known development systems, such as conventional magnetic brush development and jumping single-component development, interact with the image on the receiver, a previously toned image will be scavenged by subsequent development if interacting development systems are used. Thus, for the IOI process, there is a need for scavengeless or noninteractive development systems.
Hybrid scavengeless development technology develops toner via a conventional magnetic brush onto the surface of a donor roll and a plurality of electrode wires are closely spaced from the toned donor roll in the development zone. An AC voltage is applied to the wires to generate a toner cloud in the development zone. This donor roll generally consists of a conductive core covered with a thin (50-200 micron) partially conductive layer. The magnetic brush roll is held at an electrical potential difference relative to the donor core to produce the field necessary for toner development. The toner layer on the donor roll is then disturbed by electric fields from a wire or set of wires to produce and sustain an agitated cloud of toner particles. Typical AC voltages of the wires relative to the donor are 700-900 Vpp at frequencies of 5-15 kHz. These AC signals are often square waves, rather than pure sinusoidal waves. Toner from the cloud is then developed onto the nearby photoreceptor by fields created by a latent image.
Two-Component developer typically consists of 5-15 micron insulating toner particles, which are mixed with 50-100 micron conductive magnetic carrier granules. The developer material may comprise from about 95% to about 99% by weight of carrier and from 5% to about 1% by weight of toner.
The mixing of the developer material generates toner charge through tribo-electrification with the carrier granules.
It is well known that tribo-electrification is strongly influenced by the environmental conditions, specifically the Relative Humidity. At low RH the toner tribo-electric charge will be higher in magnitude and at high RH the toner will be lower in charge magnitude.
To maintain optimum image quality it is desirable to control the toner charge within an optimum range. To do this environmental controls are typically required to maintain the machine's ambient temperature and relative humidity. In the past, Manufacturers have put limits on the acceptable customer temperature and RH extremes before installing machines. If a location is outside the specified limits, then the customer is notified that he must install/upgrade his HVAC system or find a more suitable location.
However HVAC control is expensive and may not be available or viable in some customer locations.
One existing printing system uses electric heat pads to provide heat to each developer housing independently, allowing for station unique temperature control, see U.S. Pat. No. 6,941,089, by Rivera et al., entitled “Heating System for a Developer Housing,” issued Sep. 6, 2005.
The unique approach disclosed herein is moving the developer housing target setpoint itself as a function of ambient grains of water. This provides for increased tribo performance across a wider range of humidity conditions and for systems that experience significant changes in humidity during both operation and standby.